A hard carbon film is a hard film called diamond-like carbon (hereinafter referred to as DLC. Film/layer consisting mainly of DLC is also called DLC film/layer). Various naming is given to the hard carbon. For example, it is called a hard amorphous carbon, amorphous carbon, hard amorphous-type carbon, i-carbon, and diamond-shaped carbon. These terms are not clearly distinguished from one another.
As the essential quality of the DLC for which the above-described terms are used, the DLC has a structure in which diamond and graphite are mixed with each other and thus its structure is intermediate between that of the diamond and that of the graphite. The DLC has a high hardness almost equal to that of the diamond and is excellent in its wear resistance, solid lubricating property, thermal conductivity, chemical stability, and corrosion resistance. Therefore the DLC is utilized as protection films of dies, tools (dimension measuring jigs are included), wear-resistant mechanical parts, abrasive materials, sliding members, magnetic and optical parts. As methods of forming the DLC film, a physical vapor deposition (hereinafter referred to as PVD) method such as a sputtering method and an ion plating method; a chemical vapor deposition (hereinafter referred to as CVD) method; and an unbalanced magnetron sputtering (hereinafter referred to as UBMS) method are adopted.
The DLC film has the above-described excellent properties, whereas the DLC film generates a very large internal stress when the DLC film is formed. Although the DLC film has a high hardness and Young's modulus, it has a very small deformability. Thus the DLC film has disadvantages that it is low in its adhesiveness to a base material and liable to peel therefrom. As an art for improving the adhesiveness of the DLC film to the base material, an art of allowing the DLC film to display an excellent adhesiveness thereto even though the DLC film is formed comparatively thickly is proposed (see patent document 1). This art includes the outermost surface mainly composed of the DLC, the intermediate layer, and the base material. The base material consists of the iron-based material. The intermediate layer has the predetermined four-layer structure.
To improve the adhesiveness of the DLC film to the base material, there is proposed the art of forming the DLC film by using the graphite target and the hydrocarbon-based gas in combination as the carbon supply source and by carrying out the UBMS method under the predetermined conditions (see patent document 2).
As a concrete example of a sliding member, attempts are made to form the DLC film on raceway surfaces of bearing rings of a rolling bearing, rolling contact surfaces of rolling elements thereof, sliding contact surface of a cage thereof. Because the DLC film has the disadvantages that it is liable to peel off the base material, in forming the DLC film on the above-described surfaces of the bearing members of the rolling bearing, it is necessary to improve its adhesiveness to the surfaces of the bearing members.
To improve the adhesiveness of the DLC film to the base material by providing the hard film with the intermediate layer, there is proposed the rolling apparatus formed on the raceway groove or the rolling contact surfaces of the rolling elements made of an iron and steel material. The proposed rolling apparatus has the foundation layer which contains any one or more elements selected from among chromium (hereinafter referred to as Cr), tungsten (hereinafter referred to as W), titanium (hereinafter referred to as Ti), silicon (hereinafter referred to as Si), nickel, and iron as its composition; the intermediate layer, formed on the foundation layer, which contains the same constituent elements as those of the foundation layer and carbon such that the content rate of the carbon is larger at the side opposite to the foundation layer than at the side of the foundation layer; and the diamond-like carbon film, formed on the intermediate layer, which consists of argon and carbon such that the content rate of the argon is not less than 0.02 mass % nor more than 5 mass % (see patent document 3). To improve the adhesiveness of the DLC film to the base material by forming the intermediate layer, there is proposed the cage of the rolling bearing on which a plurality of films is formed such that the intermediate layer having the predetermined hardness is interposed between the film of the outermost layer and the cage (see patent document 4).
To improve the adhesiveness of the DLC film to the base material by an anchoring effect, there is proposed the rolling bearing in which irregularities whose height is 10 to 100 nm and average width is not more than 300 nm are formed on the raceway surface by means of ion bombardment process and the DLC film is formed on the raceway surface (see patent document 5).
In addition, there is proposed the cage composed of the hardened layer, subjected to the predetermined treatment, which is formed on the surface of the base material of the cage; the hard film, having a higher hardness than the hardened layer and coating the surface of the hardened layer therewith, which is formed on the surface of the hardened layer; and the soft film, having a solid lubrication effect and coating the surface of the hard film therewith, which is formed on the surface of the hard film. There is also proposed the method of producing this cage (see patent documents 6 and 7).